1. Field of the Invention
The present invention relates to an optical scanner using a laser diode (LD) light source and a rotary polygon mirror as a deflector and having an aperture between the light source and the rotary polygon mirror.
2. Description of the Related Art
An optical scanner having a laser diode (LD) light source is widely known in association with an optical printer, etc.
A main scan-corresponding direction and a cross scan-corresponding direction used in this specification will first be explained.
An optical arrangement of the optical scanner is developed along the optical axis of an optical element from the light source to a scanned face. An optical path developed in this optical arrangement is called a developed optical path in the following description. A light emitting section of the light source is arranged at a starting point of this developed optical path. The scanned face is arranged at a terminal point of the developed optical path. The developed optical path, a main scanning direction and a cross scanning direction are perpendicular to each other at the terminal point of the developed optical path. The main scan-corresponding direction is set to a direction parallel to the main scanning direction in an arbitrary position of the developed optical path. The cross scan-corresponding direction is set to a direction parallel to the cross scanning direction in an arbitrary position of the developed optical path.
In general, there are the following two known optical scanners using a laser diode light source and classified in accordance with optical characteristics in the main scan-corresponding direction.
In a first kind of optical scanner, a divergent light beam emitted from the laser diode light source is changed to a parallel light beam by a collimator lens as a coupling lens with respect to the main scan-corresponding direction. The parallel light beam is deflected by a rotary polygon mirror and is converged on the scanned face by an image-forming optical system. In this specification, the image-forming optical system is arranged between the rotary polygon mirror and the scanned face and is constructed by a combination system of optical elements forming a light spot on the scanned face by the deflected light beam.
In a second kind of optical scanner, a divergent light beam emitted from the laser diode light source is changed to a convergent light beam by a coupling lens with respect to the main scan-corresponding direction. This convergent light beam is then deflected by a rotary polygon mirror. The deflected light beam is further converged by an image-forming optical system and is formed as a light spot on the scanned face. For example, such an optical scanner is shown in Japanese Patent Application Laying Open (KOKAI) No. 1-302217.
The present invention can be applied to each of these first and second kinds of optical scanners.
The optical scanner using the laser diode light source and the rotary polygon mirror as a deflector has a means for correcting a so-called mirror face inclination of the rotary polygon mirror in many cases. The optical scanner having a function for correcting such a mirror face inclination is classified as follows in accordance with optical characteristics in the cross scan-corresponding direction.
A first optical scanner belongs to the first kind of optical scanner. In this first optical scanner, a parallel light beam emitted from a light source side is converged only in the cross scan-corresponding direction. A linear image extending in the main scan-corresponding direction is formed in the position of a deflecting/reflecting face of the rotary polygon mirror. A deflected light beam is focused and formed by the image-forming optical system as a light spot on the scanned face. The image-forming optical system is constructed by an anamorphic f .theta. lens for approximately providing a conjugate relation in geometrical optics with respect to positions of the deflecting/reflecting face and the scanned face in the cross-scan corresponding direction. This first optical scanner is called a first type of general optical scanner in the following description.
A second optical scanner belongs to the first or second kind of optical scanner. In this second optical scanner, a light beam emitted from the light source side is focused and formed as a linear image extending in the main scan-corresponding direction. The light beam is then formed by the image-forming optical system as a light spot on the scanned face. An anamorphic image-forming optical system approximately provides a conjugate relation in geometrical optics with respect to positions of the deflecting/reflecting face and the scanned face in the cross-scan corresponding direction. This anamorphic image-forming optical system is constructed by a spherical lens and an elongated lens. This second optical scanner is called a second type of general optical scanner in the following description. For example, as shown in Japanese Patent Publication (KOKOKU) No. 60-642, an elongated cylindrical lens is used as the elongated lens of the image-forming optical system in the second type of general optical scanner. Further, the elongated lens of the image-forming optical system in the second type of general optical scanner can be constructed by an elongated toroidal lens having a special lens face such as a barrel type toroidal lens face. The present invention can be applied to each of the first and second types of general optical scanners.
In the above-mentioned optical scanners, a portion of the light beam emitted from the laser diode light source is generally interrupted by an aperture to interrupt noise light and correct a quantity of light for performing an optical scanning operation and correct the shape of a light spot on the scanned face. An optical scanning operation of high density has recently been performed. A stable light spot having a small diameter is required to realize such an optical scanning operation of high density. The following problems are caused when the stable light spot having a small diameter is used and the above aperture is used.
In the first type of general optical scanner, a real image of the aperture is generally formed in a position separated by 100 mm or more from the scanned face by an image-forming element which is arranged as an optical element having an image-forming action between the aperture and the scanned face. Further, the scanned face is approximately arranged at infinity optically seen from a position of the aperture. Accordingly, the light spot is formed as a Fraunhofer diffraction image in each of the main scan-corresponding direction and the cross scan-corresponding direction. Therefore, a light intensity distribution with respect to the light spot is provided as a clear Gaussian distribution in each of the main and cross scanning directions. Accordingly, there is no problem about an influence of diffraction caused by the aperture.
However, since the light spot is formed as a Fraunhofer diffraction image, it is necessary to increase an opening diameter of the aperture so as to increase a size of the light beam incident to the image-forming optical system constructed by the anamorphic f .theta. lens when the diameter of the light spot is reduced to perform the optical scanning operation of high density.
The anamorphic f .theta. lens is arranged in a position separated from the scanned face so that the incident light beam has a large diameter and strict accuracy in the lens face is required. When the size of the incident light beam is further increased as mentioned above to reduce the diameter of the light spot, stricter accuracy in the lens face is required. Therefore, in consideration of cost of the optical scanner, it is not preferable to reduce the diameter of the light spot by improving the accuracy in the lens face of the image-forming optical system in the first type of general optical scanner.
In the second type of general optical scanner, a conjugate magnification .beta. in the cross scan-corresponding direction between the deflecting/reflecting face and the scanned face generally satisfies .beta.&lt;&lt;1. The real image of the aperture is formed in the vicinity of the scanned face in accordance with an arrangement position of the aperture. Accordingly, the light spot is influenced by diffraction caused by the aperture. Therefore, the light intensity distribution of the light spot becomes complicated and the diameter of the light spot is greatly changed by so-called defocus so that no diameter of the light spot is stabilized.